Naomi Watts snubbed the lead role in Diana "twice" before accepting the part.
The 44-year-old actress admitted she was worried it would be career suicide but couldn't bring herself to completely walk away from the biopic after she first rejected it.
Speaking to the Sunday Times, she said: "I thought that too, believe me. And I turned it down twice.
"I was a little bit torn, but once I'd said no, I wasn't completely at peace."
While the star usually feels content after turning down a role, she explained this felt different and she was drawn towards learning more about Princess Diana.
She added: "Sometimes when you say no, you feel free, but this just wasn't the case. There was something very intriguing about this woman's life. It was extraordinary.
"She always lived a privileged life, but there was nothing ordinary about the construct she moved into. I liked the idea that there was this fame - no one at that level can really survive it, I think. It's a very dark thing, but it's kind of true.
"I think there's also something going on with playing real-life characters. People are drawn to that, and audiences. For actors, it's definitely interesting."
Diana is due to open on Friday, 20 September.